1. Field of the Invention
The present invention relates to an image correction system and image correction method for correcting an image.
2. Description of the Related Art
Continuous use of the same printer results in variations in the output characteristics of the printer, such as output colors and gradation, due to changes in the density of ink, the ink discharge characteristics of the heads, or the like with the passage of time. A technique called “calibration” has been developed in order to stabilize the output characteristics irrespective of the above-mentioned changes with time. With the technique, the amounts of such output characteristics are measured at regular intervals, calibration information is computed from the measured values, and correction is performed based on the computed information, which enables a printer to be used with stable output characteristics. Since such calibration information is computed at regular intervals, holding computed calibration information and changing information as necessary into past information are also considered (Japanese Patent Laid-Open No. 2000-301773).
Conventional printers use only CMYK color ink such as cyan (C) ink, magenta (M) ink, yellow (Y) ink, black (K) ink, light cyan (LC) ink, and light magenta (Lm) ink. However, for higher image quality in printers, inks called spot color inks, such as gray (Gr) ink, red (R) ink, green (G) ink, and blue (B) ink, are also used to increase the number of colors to fall within the range of approximately eight to twelve. Additionally, in some cases, in consideration of, for example, a difference in the rate of ink absorption into the surface between glossy paper and non-glossy paper, different types of ink, one type for glossy paper and the other for non-glossy paper, may be used as the same color ink in order to produce better output. In other cases, the number of inks to be used may vary depending on whether importance is placed on image quality or printing speed during printing. For example, for high-quality printing, spot color inks or the like are used so as to perform printing with a great number of inks.
After the creation of such calibration information, color shifts occur due to changes in the density of the ink or the amount discharged from heads with the passage of time. It is thus desirable that calibration information be created at regular intervals. Conventionally, calibration information has been created at the same time for every ink used in a printer, so that the information on every ink has been updated at regular intervals. However, with increasing image quality and number of colors in printers, available inks vary depending on each printing medium and the quality thereof, so that all inks may not be used for a single printing medium. In such a case, calibration information on every ink cannot be created at one time.
In the case of printing a printing medium that uses any ink other than those with which calibration information has already been obtained from the last computation, if calibration information is computed every time immediately before printing, information on every ink to be used is always up-to-date and the same accuracy as usual can thus be obtained. Such computation of calibration information is, however, undesirable because it takes time and requires effort on the part of the user.
Thus, in the case of printing a printing medium that uses any ink other than those with which calibration information has already been obtained from the last computation, it is conceivable that calibration information computed with all inks used for a printing medium to be printed is searched for from a history that has been obtained from past computations except the last computation. Then, the calibration information in the history is used.
In this case, however, the calibration information in the history is used for all inks, which may inhibit effective use of up-to-date information for some types of inks.
In addition, although the number of types of inks increases with an increase in the number of colors, the influence of calibration on visual characteristics varies depending on the type of each ink. In the case where some chromatic color inks, such as cyan (C) ink, magenta (M) ink, yellow (Y) ink, light cyan (Lc) ink, and light magenta (Lm) ink, have their calibration information shifted, their color balance is disturbed. This is particularly noticeable in gray and skin colors, to which humans are highly sensitive in terms of visual characteristics. Meanwhile, in the case where some achromatic color inks, such as black (K) ink and gray (Gr) ink, have their calibration information shifted, such a shift affects only brightness components and gives no influence on the color balance. Moreover, spot color inks, such as red (R) ink, green (G) ink, and blue (B) ink, have no influence on gray and skin colors because such spot color inks are used only in specific color areas and not used on the gray axis. That is, a shift in the color balance of chromatic color inks is noticeable in gray and skin colors, to which humans are highly sensitive in terms of visual characteristics, and is thus likely to cause image degradation. On the other hand, because shifts in brightness components are more allowable than shifts in color balance, it can be said that shifts in achromatic color inks have a small influence on visual characteristics. It can also be said that shifts in spot color inks have a small influence on visual characteristics because they affect only specific color areas.
As described, a calibration method needs to be developed in consideration of the fact that the influence of calibration on visual characteristics varies depending on the type of each ink. If calibration information on every ink cannot be computed at the same time, it is necessary to achieve a calibration method that can maintain the accuracy of calibration so as to make allowable a shift in the color balance of chromatic color inks that have the most influence on visual characteristics.